Mail relay domain problem

Hi, I'm trying to configure ISPConfig3 to relay mail for a domain to another host - no local users.
I'm getting
Recipient address rejected: User unknown in relay recipient table
when I send a test mail. Configuration is as follows:

Email routing:

domaina.com
No MX enabled, destination is set to IP of destination server

Relay recipients:

@domaina enabled.

As far as I can tell this should produce a working configuration, fqdn of server is server.domainb.com so I don't believe that's the cause of the issue.